There is known a game system which can generate an image as viewed from a given viewpoint within an object space that is a virtual three-dimensional space. Such a game system is highly popular from the fact that it can provide a so-called virtual reality. If the game system is for a roll-playing game (RPG), a player can enjoy the game by controlling a character (or object) allocated to the player to move it on a map within the object space so that the player will fight against an enemy character, dialogue with any other character or visit various cities and towns.
In such a game system, an object representing a character is configured by a plurality of polygons or free-form surfaces (which are primitive surfaces in a broad sense). The image as viewed from the given viewpoint within the object space will be generated by arranging the object (model) configured by these polygons within the object space and then performing the geometry-processing (or three-dimensional computation) with respect to the object. Thus, the image can be generated in a more realistic form as if it exists in the real world scene.
On the other hand, the field of animated cartoon makes an appeal to players by using an image similar to a cellular picture specific to the animation, rather than a realistic image actually photographed.
The game picture generated by the conventional game system can make an appeal to a person that likes the reality, but would not make an appeal to a person that likes the animation.
To overcome such a problem, the inventors have developed a game system which can generate a cellular image in real-time. The generation of the cellular image requires a process of drawing the edge line of an object including a character or the like (or a process of emphasizing the edge line).
However, it has bee found that such a process contains the following problem.
It is now assumed that an edge line having its thickness equal to one pixel is to be drawn around the outer periphery of an object. In such a case, a problem is not substantially raised if the distance between the viewpoint and the object becomes nearer (or if the object is large relative to pixels on the screen) However, if the distance between the viewpoint and the object is farther (or if the object is small relative to pixels on the screen), an unnatural image will be generated since the thickness of the edge line will unnecessarily be increased relative to the size of the object.